Objectives are to characterize the immunologic and other events occurring in the host-ectoparasite relationship. Two associations - mice to lice and ticks to rabbits - with apparent involvement of different mechanisms, are being studied. Current emphasis is on the roles of cellular (macrophages, lymphocytes, and basophils) and humoral (antibody and complement) elements of tick resistance. An antigen complex has been obtained from ticks which stimulates marked in vivo skin reactivity and in vitro lymphocytes blastogenesis of resistant animals as well as inducing immunity when injected. The heritability of the response of different strains of mice to louse infestation will be determined in inbred lines established by selection. The degree of specificity of acquired resistance of cavies and rabbits to ticks is being studied by multiple exposures to closely- and distantly-related ectoparasites. Ultrastructure and the ultrastructural cytochemical nature of the tick-feeding lesion will be examined. BIBLIOGRAPHIC REFERENCES: Nelson, W.A., Bell, J.F., Clifford, C.M., and Keirans, J.E.: Interaction of ectoparasites and their hosts. J. Med. Entomol. 13:389-428, 1977. Wikel, S.K. and Allen, J.R.: Acquired resistance to ticks. III. Cobra venom factor and the resistance response. Immunology 32: 457-466, 1977.